It was never stated why. Considering all of the other procedures thickness of the tank seems irrelevant

SCP-2127 was initially discovered by two amateur divers off the coat of Antigua, who upon encounter took a photograph and vacated the water to alert authorities. Police search boats were preparing to disperse when the Foundation was notified and quickly called it off, declaring the organism to be a hoax. The two witnesses were immediately detained along with their photographic evidence, and Foundation divers went in shortly after to retrieve the crustacean.

This should be either at the end of the description or in a addendum.

collect one of the limbs and attach it to itself.

The thing is the size of a hand. It's a bit confusing considering it's later stated to have multiple limbs attached at one time.

Why is it safe? Euclid would be the proper object class. The crab isn't very interesting anyway, there's not enough meat on the article.

It is safe in the same way that many other SCP's are safe. It poses no imminent threat, and only attacks if being photographed or has ran out of limbs to use. Do you reckon it should be put up to Euclid?

I see, you fell into the pit of "object class = danger". Object classes are the difficulty of containing a SCP, not how dangerous it is. Anyway the safe classes generally almost no maintenance. The site standards would probably have it as euclid.

Lots of biological skips are Safe. It seems like if it isn't smart enough to escape containment and the only maintenance it requires just is to keep it from dying, then it is safe. As far as the locked box test goes, if the only bad thing that happens is that the skip dies, then it's probably Safe, IMO.

It has eight curved legs, almost identical to a crab's, and it has two front claws, both 30cm in length.

None of those things sound like they are human in origin. I'm also having problems imagining how this small crab manages to attach multiple human limbs to its tiny body and then somehow still move around.

EDIT: Also 30 cm is significantly longer than a hand (about twice as long).

EDIT2: Also I'm confused about the number of limbs. You say:

has never collected more than one limb at a time

but

and the amount it already owns.

implies more than one. Also "owns" seems tone-breaking, ownership is a human social concept, I think.

Ah, I can see how it is confusing. I meant that, without any collected limbs, it is a crab (with two front claws etc), but when it amasses human limbs, they get, sort of, stuck on.
In terms of moving, when a limb is stuck on, it becomes a sort of extension of the crab (the limbs being able to move etc), so that it can basically propel itself around using the limbs.
It never collects more than one limb from each body at a time, I meant to write, meaning that it can have multiple limbs, but only takes one from each corpse. I will correct this.

Thank you again for your feedback. Do you believe that with corrections being added and more description to the SCP, that it is okay? I am really fond of this idea and I want to make it as a good as possible.

Ah, I can see how it is confusing. I meant that, without any collected limbs, it is a crab (with two front claws etc), but when it amasses human limbs, they get, sort of, stuck on.

You'd think that would be in the description. "Yup, it's a crab, Bob" "What about the freaking' human arm?" "Oh, was that important?"

In terms of moving, when a limb is stuck on, it becomes a sort of extension of the crab (the limbs being able to move etc), so that it can basically propel itself around using the limbs.

I meant that if the body is only 10 cm wide or whatever and the thing masses maybe 0.5 kg, then where does it possibly attach multiple human limbs and how does it move with limbs that mass 10x what the body does?

Thank you again for your feedback. Do you believe that with corrections being added and more description to the SCP, that it is okay? I am really fond of this idea and I want to make it as a good as possible.

Well right now it's a "sort of creature", you need to develop some more depth to this.